Haplogroup BT M91, also known as Haplogroup A1b2 (and formerly as A4, BR and BCDEF), is a Y-chromosome haplogroup. BT is a subclade of haplogroup A1b (P108) and a sibling of the haplogroup A1b1 (L419/PF712).

Ancient DNA

Later Stone Age individuals excavated at Fingira Rock, Malawi, dated to around 6100 years ago (2/2 males), and at Mount Hora, Malawi, dated to around 8000 years ago (1/1 males), all belonged to Y haplogroup BT(xCT).

Distribution

Basal BT* has not been documented in any living individuals or ancient remains. No definite examples of BT(xCF,DE) – i.e. members of BT outside the only two known branches of CT, namely haplogroups CF and DE – have been identified. In some cases, because testing is undertaken only for geographically and historically likely haplogroups, the data required to identify a precise subclade has not been collected and/or recorded. For instance, research published in 2013, regarding a sample of more than 2,000 men from different parts of Africa, included 7.5% belonging to haplogroup BT(xDE,K). These approximately 150 individuals may have included, for example: B*, unknown primary branches of haplogroups B, BT, CT or CF; haplogroup C, and/or; F(xK) (i.e. haplogroup F* plus its subclades G, H and IJ, but specifically excluding the broader haplogroup K and its subclades, such as haplogroups K*, LT, K2b*, MS, NO, P, Q and R).

Phylogenetics

The ISOGG tree since 2014 has treated M91 as the defining mutation of BT. Prior to 2002, there were in academic literature at least seven naming systems for the Y-Chromosome Phylogenetic tree. This led to considerable confusion. In 2002, the major research groups came together and formed the Y-Chromosome Consortium (YCC). They published a joint paper that created a single new tree that all agreed to use.